Laser diode drive circuit

ABSTRACT

The lasing output of a laser diode (1) is controlled by the current through the diode (1). The laser diode (1) is driven by a switched mode drive circuit in which a pulsed signal is applied to a smoothing circuit, typically including a capacitor C and an inductor L, arranged to provide a smoothed output voltage whose magnitude depends on the pulse width of the drive signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a drive circuit for a laser diode.

2. Description of the Prior Art

The lasing output of a laser diode is controlled by the current throughthe diode.

JP-A-61 256 682 discloses a system for driving a laser diodeirrespective of the polarity of the diode by separating the light outputcontrol circuit of the diode and the diode in a DC manner.

Previously, laser diodes have been driven by linear control circuits asshown in FIG. 1 of the instant application. Such a system is disclosedin e.g. Melles Griot Optics Guide 5, Chapter 20, page 29 (Power SupplyDesign (1990)).

In the prior art drive circuit of FIG. 1, an external control signalindicative of the desired lasing output of a diode 1 is applied, via anoperational amplifier 3, to a transistor 2 which acts as a variableresistance connected in series with another resistor to vary the currentthrough the diode 1 to produce the required coherent light emissionoutput power. The output of the diode is detected by a photodetector anda signal commensurate therewith is fed back to the other input of theop-amp 3 where it is compared to the external control signal and thedifference is used to adjust the current through the laser diode, so asto minimise the error between the actual and desired output values.Thus, in such drive circuits the laser diode current and voltage arecontrolled by the transistor 1 in accordance with the external controlsignal and the feedback signal, and the remaining voltage from thedirect current supply bus is dropped across the series elements (thetransistor and the resistor).

We have found that these known drive circuits for laser diodes are not,however, very efficient. Typically, at full power, the laser diode hasapproximately 2V across it and a current of 0.5 A. With, for example a5V power supply, 3V would therefore be dropped across the transistor andresistor, i.e. a power loss of 1.5 W, Thus, of the 2.5 W used, only 1 Wgets used by the laser diode which is very inefficient.

Further, since such a large amount of power is dissipated as heat in theseries elements, large heat sinks are required. This results in a bulkycircuit.

The above problems are most significant where a light source is providedwith a plurality of laser diodes whose light beams are combined toprovide a power output substantially higher than that available from asingle diode. For example, WO-92/02844, describes a bank of e.g. sixteenlaser diodes arranged in a portable light source for an optical fibresuch as a body implantable probe or retina laser treatment fibre. Insuch a device sixteen drive circuits must be provided in a compact unitand it is, therefore, important that the circuits be as efficient andcompact as possible.

Another problem with known linear laser diode drive circuits arises whenit is necessary to switch the diode off. Since most op-amps have slightoffsets and tolerance mismatches, an external control signal of zerovolts will often result in a small amplifier output signal and thissignal may be sufficient to cause the laser diode to still be activatedto provide a very low power output. In practice, in order to overcomethis problem, a negative external control signal is used when zerooutput is required. However, this is not ideal and leads to difficultieswhen a very low output or precise control is required.

Thus, known laser drive circuits as shown in FIG. 1 are typically only30 to 40% efficient or less, and therefore require a higher power inputand, due to the power loss through the transistor and resistor, largeheat sinks are required resulting in a large bulky circuit. Suchcircuits are particularly unsuitable for use in applications such asdisclosed in WO-92/02844 where the size of the elements is an importantconsideration.

SUMMARY OF THE INVENTION

It is, therefore, an object of the preferred embodiment of the presentinvention to provide a compact laser diode drive circuit wherebyelectrical power losses are minimised and precise control of the laser,even at very lower power levels or zero output, is possible.

In order to achieve the foregoing object the present invention uses aswitched mode drive circuit to drive a laser diode. Accordingly, viewedfrom a first aspect, the invention provides a drive circuit for a laserdiode characterised in that said drive circuit comprises a switched modedrive circuit electrically connected to said laser diode, wherein apulsed drive signal is applied to said switched mode drive circuit and asmoothing circuit provides a smoothed output voltage whose magnitudedepends on the pulse width of the drive signal.

Viewed from a further aspect, the invention provides a method of drivinga laser diode characterised by comprising the use of a switched modedrive circuit, said switched mode drive circuit comprising a smoothingcircuit, to which a pulsed drive signal is applied, arranged to providea smoothed output voltage whose magnitude depends on the pulse width ofthe drive signal.

The invention extends to a light source unit for use in conjunction withan optical fibre, comprising a plurality of laser diodes, optical meansfor combining the light beams produced by the diodes and for directingthe combined beam into an optical fibre, wherein each laser diode isdriven by a switched mode drive circuit contained in said unit.

While various types of switched mode drive circuits were previouslyknown, such circuits have never been used to drive laser diodes.

A switched mode drive circuit as referred to herein is a circuit inwhich a pulse width modulated input signal results, due to theemployment of a smoothing circuit typically including a capacitor and aninductor, in a smoothed, i.e., substantially constant level, diodecontrol voltage whose magnitude depends on the pulse width of the inputsignal.

Switched mode drive circuits have a much greater thermal efficiency whencompared to a circuit of FIG. 1 and, since excess voltage is not droppedacross an element in series with the diode resulting in power beingdissipated as heat, no bulky cooling elements are required thus allowingthe circuit to be much smaller.

A further advantage of the use of a switched mode drive circuit inaccordance with the present invention is that the laser diode can beswitched off precisely in response to a zero power external controlsignal, i.e., with no drive pulses there can be no current flowing inthe laser diode. Restated, the operation of the laser diode is notaffected by voltage offsets.

The use of high frequency switching is preferred and allows the inductorin the smoothing circuit to be small both in size and value thus havingonly a small energy storage capability. Once the input power to thedrive circuit is switched off, the laser's ability to continue lasingdepends on the energy storage of the inductor. Therefore, the smallerthe inductance the quicker the laser will stop lasing in response to azero command signal input. This is particularly advantageous in medicalapparatus since it is a standard requirement of all electrical medicalequipment that if a failure is detected the apparatus must react quicklyto being switched off.

It is also a requirement that medical products comply with aninternational standard (IEC 601) which requires that the system must notbecome hazardous by reason of any one component failing, and soadditional safety measures must be included to prevent this.

In the linear supply shown in FIG. 1, if the transistor fails, a shortcircuit between the diode and the resistor would result in an increasein current through the series circuit thereby causing the diode tooperate at a higher output power level. Unless this destroys the diode(which is also undesirable) the high power output is potentiallyhazardous to the patient. Until now, in order to comply with theregulation, additional safety monitoring elements have been added to thedrive circuit to avoid this problem but this all adds to the overallsize, complexity and cost of the package.

Thus, in a particularly preferred embodiment the present inventioncomprises an isolating transformer, preferably a step down transformer,isolating the driving end of the circuit from the smoothing circuit andlaser diode. A suitable circuit is a Buck step-down isolated, push-pullconverter. An isolating transformer enables the apparatus to be madeintrinsically safe against single component failure without the need foradditional safety measures.

The use of a step-down transformer also increases the efficiency of thecircuit and minimises power losses.

Previously, switched mode voltage supplies have been used to providestandard 5 or 12V supply voltages. However, in driving a laser diode thevoltage requirement is much less than this, usually around 2V.

It has been found, however, the efficiency of switched mode drivecircuits decreases dramatically for a given power output as the outputvoltage decreases. Therefore, in a preferred embodiment, the diodeswhich act as self-commutating switches, such diodes traditionally beingprovided on the output side of the transformer in a transformer isolatedpush-pull converter, are replaced by active switches, preferably FETsdriven by tertiary windings on the transformer. The current required todrive the FETs is very low and this results in very low loss.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings wherein.

FIG. 1 shows a linear laser diode drive circuit in accordance with theprior art.

FIG. 2 shows a Buck push-pull voltage converter with an isolatingtransformer as used in one embodiment of the present invention.

FIG. 3 shows a modified push-pull switched mode drive circuit inaccordance with the preferred embodiment of the invention.

FIG. 4 schematically illustrates the use of the present invention in asystem wherein the outputs of plural laser diodes are combined.

DESCRIPTION OF THE DISCLOSED EMBODIMENTS

FIG. 2 depicts a switched mode drive circuit wherein drive pulses areapplied to a driven high frequency switch defined by transistors Q₁ andQ₂, which opens and closes to cause current flow alternately in oppositedirections in the center-tapped primary winding of transformer T₁. Thisresults in a pulsed output signal being induced in the secondary windingof transformer T₁. This output signal is smoothed by an LC filtercircuit to provide an output signal whose amplitude is determined by thewidth of the input pulses. In the embodiment of FIG. 2, the smoothingcircuit, i.e., the filter, is formed by inductor L and capacitor C. Inthe FIG. 3 embodiment, the filter circuit is defined by inductor L1 andcapacitor C6. This output signal is applied across the laser diode 1.The width of the input pulses is controlled, for example in the mannershown in FIG. 1, in dependence on an external input command signalindicative of the desired output power and a feedback signal from asensor which detects the actual output of the laser diode.

In prior art linear systems, as shown in FIG. 1, all of the supplyvoltage is dropped across the diode and series elements such as atransistor and a resistor. Thus if, for example, the power supplyvoltage was 12V and, at full power, the diode voltage was 2V and theoperating current was 3 A, the overall power would be 36 W, with 30 W ofthat being dissipated as heat in the series element and only 6 W beinguseful power output from the diode. This is clearly very inefficient andalso results in the series components becoming very hot and soadditional cooling elements are required.

Using the present invention, almost all of the generated power is fromthe laser and the system is, therefore, around 90% efficient.

The use of step-down isolation transformer T₁ considerably increases theefficiency of a control in accordance with the present invention. In atypical circuit, as indicted on FIG. 3, the ratio of primary turns tosecondary turns in 47:3 turns, for a 2 V, 3 A output derived from adirect current supply of approximately 50 V which is connected to theprimary winding. In the example being described, the primary current isonly about 140 mA and thus the current related losses are greatlyreduced.

In practice a bridge rectifier (not shown) will usually be connectedbetween the mains transformer and the switching circuit, the bridgerectifier thus comprising the 49V direct current source of FIG. 3.Without the step-down transformer T₁ the diode operating current,typically 3 A, would need to be supplied from the mains transformer andthrough the bridge rectifier diodes. Since there is a 0.7V drop acrosseach diode in the bridge rectifier this would result in a high powerloss across the rectifier. However, because the step-down transformerresults in a very low current through the transformer primary winding,and thus through the rectifier, these losses are minimised. Due to thevery low current, Joule losses on all the cables are also minimised.

The use of isolating transformer T₁ is also a safety feature which, inaccordance with medical instrumentation regulations, protects againstfailure of any single component producing a hazardous situation.

Only A.C. power can get across the transformer. Therefore, if either ofthe switching transistors, Q₁, Q₂, fails, resulting in a short circuitor open circuit, no power is transferred across the transformer to thediode.

Further, the drive circuit of the present invention allows the groundreference point of the laser diode to be arbitrarily selected such that,if the transformer fails, e.g. if a short circuit occurs between theprimary and secondary windings of transformer T₁, the laser diode willbe reverse biased and thereby switched off.

In the embodiment shown in FIGS. 2 and 3, the anode of the laser diodeis connected to ground. If the transformer short circuits, since all thevoltages on the primary winding are positive, a positive potential willbe applied to the cathode of the diode which reverse biases the laserdiode.

The arbitrary choice of ground potential for the diode anode voltagelevel also enables many laser diodes to be put in thermal and electricalcontact with their anodes joined as schematically illustrated in FIG. 4.

If any of the components on the secondary side of Transformer T₁ failthe lasing power will be reduced and no hazard arises.

All of the above-discussed features are particularly advantageous inmedical instrumentation and, since additional safety elements are notneeded, the resulting package is more compact. This is an especiallyimportant consideration for drive circuits for use in the systemdisclosed in WO-A-92/02844 where sixteen laser diodes are mounted in acompact unit.

Although switched mode power supply topologies are known in otherapplications, e.g. as push-pull drive circuits for loudspeakers, theyhave not previously been used to drive laser diodes. One reason is thatin these other applications, the circuits generally provide moderate orhigh (usually 5 or 12V) output voltages, and in any case not less than5V, whereas laser diodes require a much lower control voltage, typically2V. It has been found that the losses in such circuits increasedramatically as the voltage falls below 5V.

The switched mode power supplied shown in FIG. 2 and FIG. 3 comprisestwo switches for controlling current flow through the isolationtransformer primary winding. In the FIG. 2 embodiment, switchingtransistors Q₁, Q₂ are used, and a secondary, self-commutating switch,for which diodes S₃, S₄ are used as a self-commutating switch.

The use of switched diodes, however, results in some power loss, andthis can be particularly serious in the case of a laser diode whichconstitutes a high current lower voltage load. For example, a typicalswitching diode has a 0.7V drop across it. For a load which requires 10W at 10V the current is 1 A, and the power loss across the switchingdiode is a relatively insignificant 0.7 W. However, for a load whichrequires 10 W at 1V (such as a laser diode) the current is 10 A and thepower loss 7 W.

Thus, in order to minimise these losses, in the preferred embodiment ofFIG. 3, the diodes S₃, S₄ are replaced with active synchronous switches,e.g. in the form of FETs Q₃, Q₄, which are driven by tertiary coils onthe transformer secondary winding as pulses are applied. We have foundthat this arrangement dramatically reduces power loss in a laser diodedrive circuit.

It is preferable to minimise switching losses as much as possible byusing a low-leakage inductance transformer In the preferred embodimentthe Transformer T₁ is formed by winding a first primary winding ofbifilar wire very tightly and neatly around the core. A secondarywinding is then wound over this, followed by a second primary windingand then a tertiary winding. The primary is split into two halves. Lowleakage inductance is achieved by getting good coupling between theprimary and secondary windings. The secondary winding should bephysically close and entirely cover the primary winding. A suitablenumber of turns for each winding in accordance with one reduction topractice of the invention is specified by way of example in FIG. 3.

Although switched mode power supplies are known, their use in drivinglaser diodes, particularly in medical applications, requires specialconsiderations as regards efficiency, size and safety as discussedabove.

FIG. 4 schematically illustrates the use of the present invention in asystem where a plurality of laser diodes, with common anode potential,are used in conjunction with an optical fiber, means being provided forcombining the light beams produced by the individual diodes and fordirecting the combined beam into the optical fiber. While FIG. 4 depictsonly a pair of laser diodes 1, in a medical application such as shown inabove-referenced WO-2/02844, there may be a bank of sixteen diodes whichare simultaneously operated to provide the requisite output power.

It will be appreciated that the illustrated embodiments are given onlyby way of example. Other suitable forms of switched mode power supplywill be known to those of relevant skill and could be used in theinvention.

We claim:
 1. A switched mode drive circuit for a laser diodecomprising:a source of direct current, said current source havingterminals at different potentials; a step-down isolation transformerhaving primary and secondary windings, said primary winding beingconnected to a first terminal of said source; first switch meansconnected between said primary winding and a second terminal of saidsource for controlling current flow through said primary winding; meansproviding pulse width modulated control signals to said first switchmeans to cause periodic current flow from said source through saidtransformer primary winding whereby a pulsating voltage is induced insaid transformer secondary winding; filter means responsive to voltagepulses applied thereto for providing a substantially constant outputvoltage having a magnitude dependent on the width of said appliedvoltage pulses; synchronous self-commutating switch means connectedbetween said secondary winding of said transformer and said filtermeans, voltages induced in said transformer secondary winding beingapplied to said filter means by said synchronous switch means wherebythe magnitude of said output voltage is commensurate with the width ofsaid control signals; and means for delivering said filter means outputvoltage to a laser diode to control the coherent light generated by saidlaser diode.
 2. The drive circuit of claim 1 wherein said filter meanscomprises a capacitor and an inductor.
 3. The drive circuit of claim 2wherein said transformer, first switch means, means providing pulsewidth modulated control signals, filter means and synchronous switchmeans cooperate to define a transformer isolated push-pull converter. 4.The drive circuit of claim 3 wherein said synchronous switch meanscomprises at least first and second field effect transistors, theswitching control signals for said field effect transistors beingderived from voltages induced in secondary windings of said transformer.5. The drive circuit of claim 2 wherein said synchronous switch meanscomprises at least first and second field effect transistors, theswitching control signals for said field effect transistors beingderived from voltages induced in secondary windings of said transformer.6. The drive circuit of claim 1 wherein said transformer, first switchmeans, means providing pulse width modulated control signals, filtermeans and synchronous switch means cooperate to define a transformerisolated push-pull converter.
 7. The drive circuit of claim 6 whereinsaid synchronous switch means comprises at least first and second fieldeffect transistors, the switching control signals for said field effecttransistors being derived from voltages induced in secondary windings ofsaid transformer.
 8. The drive circuit of claim 1 wherein saidsynchronous switch means comprises at least first and second fieldeffect transistors, the switching control signals for said field effecttransistors being derived from voltages induced in secondary windings ofsaid transformer.
 9. A light source for use in conjunction with anoptical fiber, said light source comprising:a plurality of laser diodes;means for controlling the energization of individual of said laserdiodes, said energization controlling means each comprising: a switchedmode drive circuit for a laser diode comprising:a source of directcurrent, said current source having terminals at different potentials; astep-down isolation transformer having primary and secondary windings,said primary winding being connected to a first terminal of said source;first switch means connected between said primary winding and a secondterminal of said source for controlling current flow through saidprimary winding; means providing pulse width modulated control signalsto said first switch means to cause periodic current flow from saidsource through said transformer primary winding whereby a pulsatingvoltage is induced in said transformer secondary winding; filter meansresponsive to voltage pulses applied thereto for providing asubstantially constant output voltage having a magnitude dependent onthe width of said applied voltage pulses; synchronous self-commutatingswitch means connected between said secondary winding of saidtransformer and said filter means, voltages induced in said transformersecondary winding being applied to said filter means by said synchronousswitch means whereby the magnitude of said output voltage iscommensurate with the width of said control signals; and means forcombining the light beams produced by said laser diodes and fordirecting the combined light beam into an optical fiber.
 10. The drivecircuit of claim 9 wherein said filter means comprises a capacitor andan inductor.
 11. The drive circuit of claim 10 wherein said transformer,first switch means, means providing pulse width modulated controlsignals, filter means and synchronous switch means cooperate to define atransformer isolated push-pull converter.
 12. The drive circuit of claim11 wherein said synchronous switch means comprises at least first andsecond field effect transistors, the switching control signals for saidfield effect transistors being derived from voltages induced insecondary windings of said transformer.
 13. A method for exercisingcontrol over the output power of a laser diode comprising the stepsof:generating, from a direct current source, voltage pulses having awidth commensurate with the desired light output intensity of the laserdiode; producing a substantially constant voltage having a magnitudewhich is dependent upon the width of said generated voltage pulses;direct current isolating said substantially constant voltage from thedirect current source; and applying the substantially constant voltageacross the laser diode.
 14. The method of claim 13 wherein said step ofproducing a substantially constant voltage comprises passively filteringsaid generated voltage pulses.
 15. The method of claim 14 wherein saidstep of generating voltage pulses includes synchronously switchingpulses derived from a direct current to alternating current converterprior to passive filtering.